Physiological role of beta-phosphoglucomutase in Lactococcus lactic

A beta -phosphoglucomutase (beta -PGM) mutant of Lactococcus lactic subsp. lactis ATCC 19435 was constructed using a minimal integration vector and do uble-crossover recombination. The mutant and the wild-type strain were grow n under controlled conditions with different sugars to elucidate the role o f beta -PGM in carbohydrate catabolism and anabolism. The mutation did not significantly affect growth, product formation, or cell composition when gl ucose or lactose was used as the carbon source. With maltose or trehalose a s the carbon source the wild-type strain had a maximum specific growth rate of 0.5 h(-1), while the deletion of beta -PGM resulted in a maximum specif ic growth rate of 0.05 h(-1) on maltose and no growth at all on trehalose. Growth of the mutant strain on maltose resulted in smaller amounts of lacta te but more formate, acetate, and ethanol, and approximately 1/10 of the ma ltose was found as beta -glucose 1-phosphate in the medium. Furthermore, th e beta -PGM mutant cells grown on maltose were considerably larger and accu mulated polysaccharides which consisted of alpha -1,4-bound glucose units. When the cells were grown at a low dilution rate in a glucose and maltose m ixture, the wild-type strain exhibited a higher carbohydrate content than w hen grown at higher growth rates, but still this content was lower than tha t in the beta -PGM mutant. In addition, significant differences in the init ial metabolism of maltose and trehalose were found, and cell extracts did n ot digest free trehalose but only trehalose 6-phosphate, which yielded beta -glucose 1-phosphate and glucose 6-phosphate. This demonstrates the presen ce of a novel enzymatic pathway for trehalose different from that of maltos e metabolism in L. lactis.